Relaxed InGaAs buffers

ABSTRACT

In x Ga 1−x As structures with compositionally graded buffers grown with organometallic vapor phase epitaxy (OMPVE) on GaAs substrates. A semiconductor structure and a method of processing such a structure including providing a substrate of GaAs; and epitaxially growing a relaxed graded layer of In x Ga 1−x As at a temperature ranging upwards from about 600° C.

PRIORITY INFORMATION

[0001] This application is a continuation-in-part of Ser. No. 09/198,960filed Nov. 24, 1998, which claims priority from provisional applicationSer. No. 60/067,189 filed Dec. 1, 1997.

BACKGROUND OF THE INVENTION

[0002] The invention relates to the field of lattice-mismatched epitaxy,and in particular to the field of creating lattice-mismatched devicesbased on relaxed InGaAs alloys.

[0003] Most electronic and optoelectronic devices that require layersdeposited by epitaxial growth utilize lattice-matched epitaxial layers,i.e. the crystal structure of the layer has the same lattice constant asthat of the substrate. This lattice-matching criterion has beenimportant in creating high quality materials and devices, sincelattice-mismatch will create stress and in turn introduce dislocationsand other defects into the layers. The dislocations and other defectswill often degrade device performance, and more importantly, reduce thereliability of the device.

[0004] The applications of lattice-mismatched layers are numerous. Inthe InGaAs material system, one important composition is in the range of20-30% In. These compositions would allow the fabrication of 1.3 μmoptical devices on GaAs substrates, as well as high electron mobilitytransistors with superior performance on GaAs substrates. Alloys in thedesired composition range are lattice-mismatched to GaAs and InPsubstrates, and therefore usually suffer from high dislocationdensities. One known method to minimize the number of dislocationsreaching the surface of a relaxed, mismatched layer is tocompositionally grade the material (in this case through grading the Incomposition) so that the lattice-mismatched is reduced over a greatthickness.

[0005] Compositional grading is typically accomplished in InGaAs alloysby grading the In composition at a low temperature of growth, typicallyless than 500° C. The dominant technique for depositing these relaxedlayers in the InGaAs system has been molecular beam epitaxy (MBE). TheMBE has a limited growth rate, and therefore the growth of these relaxedbuffers is tedious and costly. A supply of virtual InGaAs substrates(i.e., a GaAs substrate with a relaxed InGaAs layer of high quality atthe surface) would be in demand commercially, since the user could buythe substrate and deposit the device layers without having to depositthe graded InGaAs layer. To create a supply of these wafers at low cost,metalorganic chemical vapor deposition (MOCVD) offers greater potential.

[0006] There have been no successful reports of high quality relaxedgraded InGaAs layers grown by MOCVD. There are fundamental materialsproblems with InGaAs graded layers grown in a certain temperaturewindow. Thus, most attempts to grow relaxed layers with MOCVD have mostlikely failed for attempting to grow the layers under standardconditions, i.e. temperatures in the deleterious window.

SUMMARY OF THE INVENTION

[0007] It is therefore an object of the invention that with theappropriate grading rate, there is an unforeseen higher temperaturewindow, which can be accessed with MOCVD and not MBE, in which highquality relaxed InGaAs alloys can be grown. Relaxed InGaAs grown withMOCVD in this temperature range have the economic advantages of usingthe MOCVD technique, as well as creating completely relaxed InGaAslayers of high quality.

[0008] Another object of the invention is to allow the fabrication ofrelaxed high quality InGaAs alloys on GaAs substrate with the MOCVDdeposition technique. These virtual InGaAs substrates can be used in avariety of applications, in particular 1.3 μm optical devices andhigh-speed microwave transistors can be fabricated on such substrates.It is a further object of the invention to disclose the appropriateconditions during growth in which it is possible to achieve high qualitymaterial and devices using this InGaAs/GaAs.

[0009] In_(x)Ga_(1−x)As structures with compositionally graded buffersgrown with organometallic vapor phase epitaxy (OMPVE) on GaAs substratesand characterized with plan-view and cross-sectional transmissionelectron microscopy (PV-TEM and X-TEM), atomic force microscopy (AFM),and x-ray diffraction (XRD). Surface roughness experiences a maximum atgrowth temperatures near 550° C. The strain fields from misfitdislocations induce a deleterious defect structure in the <110 >directions. At growth temperatures above and below this temperature, thesurface roughness is decreased significantly; however, only growthtemperatures above this regime ensure nearly complete relaxed gradedbuffers with the most uniform composition caps and highest qualitymaterial. With the optimum growth temperature for gradingIn_(x)Ga_(1−x)As determined to be 700° C., it was possible to produceIn_(0.33)Ga_(0.67)As diodes on GaAs with threading dislocation densities<8.5×10⁶/cm².

[0010] Accordingly, the present invention provides a method ofprocessing semiconductor materials, including providing a substrate ofGaAs; and epitaxially growing a relaxed graded layer of In_(x)Ga_(1−x)Asat a temperature ranging upwards from about 600° C.

[0011] The present invention also provides a semiconductor structureincluding a substrate of GaAs, and a relaxed graded layer ofIn_(x)Ga_(1−x)As which is epitaxially grown at a temperature rangingupwards from about 600° C.

[0012] These and other objects, features and advantages of the presentinvention will become apparent in light of the following detaileddescription of preferred embodiments thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a simplistic block diagram of a semiconductor structurein accordance with the invention;

[0014]FIG. 2 is a graph illustrating RMS roughness for structures grownat different temperatures;

[0015]FIG. 3 is a table which shows the growth temperature, composition,and grading rate for nominal x_(In)=0.06 structures;

[0016]FIG. 4 is a X-TEM micrograph for a x_(In)=0.06 In_(x)Ga_(1−x)Asstructure grown at 550° C.;

[0017]FIG. 5 is a X-TEM micrograph for a x_(In)=0.06 In_(x)Ga_(1−x)Asstructure grown at 700° C.;

[0018]FIG. 6 is a PV-TEM micrograph of a x_(In)=0.06 structure grownat550° C. (g=<220>);

[0019]FIG. 7 is an X-TEM micrograph of a x_(In)=0.33 In_(x)Ga_(1−x)Asstructure grown at 700° C.; and

[0020]FIG. 8 is a simplistic block diagram of a semiconductor structureof the invention with a device configured thereon.

DETAILED DESCRIPTION OF THE INVENTION

[0021] Compositionally graded buffers are implemented in latticemismatched heteroepitaxy to maintain a low threading dislocation densityand to achieve a completely relaxed growth template. Organometallicvapor phase epitaxy (OMVPE) is a well-established growth technique thatis capable of growth rates that are significantly greater than growthrates in molecular beam epitaxy (MBE). Therefore, OMVPE is a morepractical growth tool for fabricating graded buffers. The ability togrow In_(x)Ga_(1−x)As graded buffers with OMVPE facilitates themanufacture of commercial lattice-mismatched devices, including 1.3 μmwavelength emitting lasers on GaAs.

[0022] In accordance with the invention, In_(x)Ga_(1−x)As graded buffersare grown on GaAs substrates with atmospheric OMVPE at different growthtemperatures. FIG. 1 is a simplistic block diagram of a semiconductorstructure 100 including a substrate 102 of GaAs on which is grown arelaxed graded layer 104 of In_(x)Ga_(1−x)As. Exemplary samples weregrown in a Thomas Swan atmospheric research reactor on n⁺GaAssubstrates. The buffers were characterized with plan-view andcross-sectional transmission electron microscopy (PV-TEM and X-TEM),atomic force microscopy (AFM), and x-ray diffraction (XRD). The PV-TEMand X-TEM characterization was done with a JEOL 2000FX microscope. TheXRD was performed with a Bede D³ triple axis diffractometer. The AXMexperiments were conducted with a Digital Instruments D3000 Nanoscope.

[0023] In order to explore graded In_(x)Ga_(1−x)As relaxation, exemplarysamples were graded to x_(In)≈0.06 (≈0.4% mismatch). Such a small amountof mismatch should produce excellent relaxed layers independent of mostgrowth parameters.

[0024] Undoped In_(x)Ga_(1−x)As graded buffers with nominal final indiumconcentration of x_(In)=0.06 were grown at temperatures between 500-700°C. In addition, x_(In)=0.15 and x_(In)=0.33 graded buffers were grown at700° C. All growths were performed with a 5000 sccm H₂ carrier flow and134 sccm AsH₃ flow. The trimethylgallium (TMG) flow was fixed at 0.030sccm throughout the graded buffer growth sequence. Compositional gradingwas accomplished by stepping the trimethylindium (TMI) flow rate byapproximately 0.005 sccm up to a final flow of 0.031 sccm for thex_(In)=0.06 graded buffer, 0.077 sccm for the X_(In=)0.15 graded buffer,and 0.163 sccm for the x_(In)=0.33 graded buffer. Sufficient vent timeswere incorporated after each change in TMI flow setting to ensure theexpected composition, during which time the sample was kept at thegrowth temperature. All samples, except the structure with thex_(In)=0.33 graded buffer, had an undoped 1 μm uniform cap layer 106.The sample with a x_(In)=0.33 graded buffer had a 2 μm cap whichincorporated a PIN diode structure.

[0025] A visual inspection of the surface morphology reveals a strongdependence on growth temperature, a surprising result for such a lowlattice mismatch. AFM surface topology data taken on 10 μm×10 μm areasof each sample, including the x_(In)=0.15 and x_(In)=0.33 structures isdepicted in the graph of FIG. 2. The data show that the rms roughnessfor the nominal x_(In)=0.06 sample grown at 550° C. has a significantlygreater rms roughness value (52 nm) than the other structures which havean rms value of about 10 nm. In fact, despite the low mismatch, thesample grown at 550° C. is not specular to the eye, i.e., it is visiblyrough. In addition, the surface roughness for the equivalent x_(In)=0.33structure grown at 770° C. was less than the surface roughness ofx_(In)=0.06 structures grown at all lower temperatures.

[0026] A criterion for applications is that there must be a great amountof strain relief and a low threading dislocation density. To determinethe degree of strain relaxation and the indium composition, glancingexit (224) reciprocal space maps were conducted with triple axis XRD.Since the x_(In)=0.06 structures were of low mismatch and relativelythin (2 μm) the effect of epilayer tilt was expected to be negligible,and thus no glancing incidence (224) or (004) reciprocal space maps wereacquired to extract this effect.

[0027] X-TEM was used to measure the thickness and in combination withthe final composition, the grading rate. FIG. 3 is a table which showsthe growth temperature, composition, and grading rate for the nominalx_(In)=0.06 structures. The table shows that the indium compositionsteadily increased with decreasing temperature (with the exception ofthe structure grown at 600° C.), which is due to the lower crackingtemperature for TMI. In addition, the growth rate decreased withdecreasing temperature, which in turn provided for a higher grading rateat lower temperature. It should be noted that there is a small error(≈300 Å) in the measurement of the graded buffer thickness due to thecalibration of the TEM and the tilting of the TEM specimens.

[0028]FIG. 3 shows the residual strain in each of the nominalx_(In)=0.06 structures as a function temperature. The structure grown at500° C. had a noticeably greater residual strain left in the structure,and there is no general trend among the other samples. However, sincethe compositions and grading rates differed, the efficiency of thegraded buffers at relieving strain was compared after calculating theequilibrium plastic strain rate (strain/thickness) and the overallequilibrium plastic strain.

[0029] The equilibrium plastic rate is given by $\begin{matrix}{{C_{\delta}(h)} = {C_{f} + \frac{3{D\left( {1 - \frac{v}{4}} \right)}\quad \ln \quad \left( \frac{2\pi \quad b\quad C_{\delta}}{e} \right)}{2{Yh}^{2}}}} & (1)\end{matrix}$

[0030] where C_(f) is the mismatch introduction rate (misfit/thickness),Y is the Young's modulus, h is the film height and D=Gb/[2p(1−-ν)] withG, the shear modulus; ν, Poisson's ratio; and b, the magnitude of theBergers vector (60° dislocations are assumed). The expression for theoverall plastic deformation in a graded buffer is:

δ_(eq)(h)=C₆₇ (h)h  (2)

[0031] The percentage of the equilibrium strain relieved (i.e., percentrelaxation) is also listed in the table of FIG. 3. All the samplesshowed a similar degree of relaxation (≈80-85 %). At such a low mismatchit is difficult to distinguish the most effective growth conditions forstrain relief. The disparities in strain relief were expected to be morepronounced at higher indium compositions. In general, higher growthtemperatures would allow for more efficient strain relief.

[0032] The X-TEM and XRD data exhibit differences in microstructurebetween the x_(In)=0.06 sample grown at 550° C. and the same structuregrown at 700° C., in agreement with the drastic difference in surfacemorphology. FIGS. 4 and 5 show the X-TEM micrographs of these twostructures. Both structures have threading dislocation densities belowthe X-TEM limit (<10⁸/cm²). Thus, the very poor structure morphology ofthe 550° C. sample is not due to a very high defect density in the topIn_(x)Ga_(1−x)As layer. The uniform cap layer of the structure grown at550° C. does show-additional semi-circular regions in the top of thefilm. These features are not dislocations, as the contrast is weak, andare believed to be variations in strain from neighboring regions thatmay have undergone phase separation during growth. However, recentstudies show that high-energy defects are formed in this temperaturerange, and their origin is uncertain. Regardless, growth temperatures inthis range near 550° C. lead to rough surface morphology which laterleads to high threading dislocation densities.

[0033] A (224) glancing exit reciprocal space map of this structureshows a significantly greater spread in the 20 direction for the uniformcap than any of the other samples grown at different temperatures. TheFWHM data for the XRD peaks from the uniform caps are listed in thetable of FIG. 3. It will be appreciated that the sharpest peak in the 20direction is from the sample grown at 700° C. The spread in the 20direction for the 550° C. sample is consistent with a spread in latticeconstant due to indium composition variations or defect formations. Inaddition, the spread in the ω direction (FWHM data also tabulated in thetable of FIG. 3) for the cap in the structure grown at 550° C. was lessthan that of the other x_(In)=0.06 samples, creating a circularprojection of the (224) spot in reciprocal space, as opposed to thetypical elliptical spot expected from a high quality relaxedheterostructure.

[0034]FIG. 6 shows a plan view TEM image of the x_(In)=0.06In_(x)Ga_(1−x)As structure grown at 550° C. showing striations under ag=<220> diffraction condition in <110 > directions (the same directionas the dislocations in the graded buffer), which may be attributed tocompositional variations due to phase separation and/or defect formationat the boundaries between regions. These striations disappear under theother g=<220 > condition and g=<400 > conditions.

[0035] The data suggest that there is a correlation between thedislocations in the graded buffer and the observed boundary defects,which in turn cause the drastic surface roughening in the temperatureregime where phase separation is favored. In the low temperature growthregime (500° C.), the surface roughening is kinetically limited, as theatoms do not have the mobility to attach to sites which cause the longrange variations that are seen at higher growth temperatures. In thehigh temperature growth regime (>600° C.), thermodynamics dictate thegrowth conditions, as the growth occurs above the point at which suchboundary defects are favorable.

[0036] At the growth temperatures within the range 500-600° C., thestrained regions with boundaries form, as shown in FIG. 6. Thesefeatures occur along the <110 > direction since the misfit dislocations,their strain fields, and their cross-hatch surface lie along the <110>directions. As a consequence, the In_(x)Ga_(1−x)As layers roughen assurface energy is created and as strain energy is relieved. It will beappreciated that this roughening (i.e., very pronounced cross-hatchpattern), is much more severe than the roughening that occurs in systemswhich lack this apparent phase instability.

[0037] Although the X-TEM image of the x_(In)=0.06 structure grown at550° C. did not show threading dislocations, it has been shown in theSi_(x)Ge_(1−x) materials system that a rough surface with increasinglattice mismatch will eventually lead to a high threading dislocationdensity even in structures with slow grading rates, as described inSamavedam et al., “Novel Dislocation Structure and Surface MorphologyEffects in Relaxed Ge/SiGe/Si Structures”, J. Appl. Phys., 81(7), 3108(1997), incorporated herein by reference. Grading to greater indiumcompositions at 550° C. produces very rough surfaces that eventuallylead to high threading dislocation densities. Although surface roughnesscan be decreased by growing at lower temperature, this cannot beachieved without compromising the relaxation of the graded buffer. Thisimplies that the only window for growth of In_(x)Ga_(1−x)As gradedbuffers which provides both relaxation and good surface morphology is athigh temperature. It is important to note that MBE can not attain suchhigh growth temperatures due to limited arsenic overpressure.

[0038] With surface roughness and relaxation conditions determined to beoptimum at a growth temperature of 700° C., a x_(In)=0.33 devicestructure was grown. FIG. 7 is an X-TEM micrograph of this structurewith no threading dislocations within the X-TEM limit. No threadingdislocations could be observed in PV-TEM, showing that the threadingdislocation density in this structure is <8.5×10⁶/cm² given this size ofviewable area in TEM. (224) glancing incidence and glancing exit θ/2θdouble axis XRD scans showed the composition to be x_(In)=0.33 and thestructure was 99.39% relaxed.

[0039] Accordingly, it has been demonstrated that growth ofIn_(x)Ga_(−x)As graded buffers is sensitive to the growth temperature.In the temperature range approximately between 500 and 600° C., thesurface morphology of the structures degrades and rapidly leads to poorquality layers, even at relatively low mismatch. Only growth at highertemperatures produces relaxed layers with sufficient relaxation, goodsurface morphology, and low threading dislocation densities. The lowthreading dislocation densities are sufficient for the fabrication ofelectronic and optoelectronic devices such as 1.3 μm wavelength emittinglasers.

[0040] As shown in FIG. 8, once a virtual substrate of In_(x)Ga_(1−x)Asis created, new optoelectronic and electronic devices can be configuredon top of the structure. For example, analogous to heterostructuresgrown on GaAs substrate, heterostructure devices composed of other alloycompositions can now be fabricated on these substrates. Layers 808, 810,812 provided on the structure 800 can be layers of theAl_(y)(In_(x)Ga_(1'x))_(1−y)As or the In_(x)Ga_(1−x)As_(−y)P_(y) alloyfamilies. These alloys can be used to create 1.3 μm heterostructurelayers on GaAs substrates, a great advantage over the 1.3 μm devicesthat need to be grown on InP substrates due to the lattice-matchingcriterion. In the illustrated embodiment, layers 808 and 812 are thecladding layers of a heterostructure laser, and 810 is the activeregion. In_(x)Ga_(1−x)P is often chosen for the cladding layers sincethe absence of Al results in better device reliability.

[0041] Similar heterostructures from the Al_(y)(In_(x)Ga_(1−x))_(1−y)Asalloy system on these virtual In_(x)Ga_(1−x)As buffer layers can befabricated into high transconductance field effect transistors (FETs)useful in microwave applications. The virtual substrate allows theelectron channel or active region (810 in FIG. 8) to be composed of muchhigher In concentrations than FETs grown on GaAs without the virtualbuffer. The higher In concentration in the channel results in very highelectron mobilities, and the large band off-sets that can be created inthe Al_(y)(In_(x)Ga_(1−x))_(1−y)As heterostructure system (810-812)create a very high electron concentration in the channel. Thus, the highmobility and high electron concentrations lead to transistors with veryhigh transconductance.

[0042] Even greater quality buffers can be created through combining theinvention with other known improvements for graded layers, such as theuse of planarization techniques to lower defect densities in gradedbuffers.

[0043] Although the present invention has been shown and described withrespect to several preferred embodiments thereof, various changes,omissions and additions to the form and detail thereof, may be madetherein, without departing from the spirit and scope of the invention.

[0044] What is claimed is:

1. An integrated circuit comprising a heterostructure for a 1.3 μmwavelength laser, said heterostructure comprising a GaAs substrate, arelaxed graded layer of In_(x)Ga_(1−x)As which is epitaxially grown onsaid substrate at a temperature ranging upwards from about 600° C., andsemiconductor device layers.
 2. The integrated circuit of claim 1 ,wherein a uniform layer of In_(y)Ga_(1−y)As is positioned between saidrelaxed graded layer and said semiconductor device layers.
 3. Theintegrated circuit of claim 1 , wherein said semiconductor device layersare comprised of a first cladding layer, an active layer, and a secondcladding layer.
 4. The integrated circuit of claim 3 , wherein saidfirst cladding layer comprises Al_(z)(In_(w)Ga_(1−w))_(1−z)As, saidactive layer comprises Al_(t)(In_(u)Ga_(1−u))_(1−t)As, and said secondcladding layer comprises Al_(v)(In_(r)Ga_(1−r))_(1−v)As.
 5. Theintegrated circuit of claim 4 , wherein z is approximately equal to vand w is approximately equal to r.
 6. The integrated circuit of claim 3, wherein said first cladding layer comprisesIn_(w)Ga_(1−w)As_(1−z)P_(z), said active layer comprisesIn_(t)Ga_(1−t)As_(1−u)P_(u), and said second cladding layer comprisesIn_(v)Ga_(1−v)As_(1−r)P_(r).
 7. The integrated circuit of claim 6 ,wherein z and r are approximately equal to
 1. 8. The integrated circuitof claim 1 , wherein said relaxed graded layer is epitaxially grown at atemperature of approximately 700° C.
 9. The integrated circuit of claim1 , wherein said relaxed graded layer is epitaxially grown withorganometallic vapor phase epitaxy (OMVPE).
 10. The integrated circuitof claim 1 , wherein the degree of relaxation of said relaxed gradedlayer is from about 90% to 100% for x>0.25.
 11. The integrated circuitof claim 8 , wherein said relaxed graded layer is epitaxially grownwhile increasing the content of In at a gradient of less than about 15%per micron to a final composition in the range of 0.1<x<1.0.
 12. Anintegrated circuit comprising a heterostructure for a field-effecttransistor (FET), said heterostructure comprising a GaAs substrate, arelaxed graded layer of In_(x)Ga_(−x)As which is epitaxially grown onsaid substrate at a temperature ranging upwards from about 600° C., andan active layer.
 13. The integrated circuit of claim 12 , wherein auniform layer of In_(y)Ga_(1−y)As is positioned between said relaxedgraded layer and said active layer.
 14. The integrated circuit of claim12 , wherein said active layer comprises Al_(z)(In_(w)Ga_(1−w))_(1−z)As.15. The integrated circuit of claim 12 , wherein said relaxed gradedlayer is epitaxially grown at a temperature of approximately 700° C. 16.The integrated circuit of claim 12 , wherein said relaxed graded layeris epitaxially grown with organometallic vapor phase epitaxy (OMVPE).17. The integrated circuit of claim 12 , wherein the degree ofrelaxation of said relaxed graded layer is from about 90% to 100% forx>0.25.
 18. The integrated circuit of claim 15 , wherein said relaxedgraded layer is epitaxially grown while increasing the content of In ata gradient of less than about 15% per micron to a final composition inthe range of 0.1<x<1.0.